Sometimes abbreviated as "high as G.P.", this phrase can be used in general to describe anything that is very tall or high-positioned (physically). It is sometimes also used figuratively when describing being "high" (under the influence of a substance, i.e. marijuana).

Man, I'm not getting up on that ladder. That thing is as high as G.P.

Dude, how much of that sh*t did you smoke. **I don't know man, but I'm higher than giraffe pussy...**